My YS 60SR/Hatori Sab1 has been running great i have 7 gallons through it. I've been running cool power 15% YS blend made for the 120 and also cool power 30%. A couple days ago it was flying great then went super rich mid flight, it was still making some power but there was a noticeable difference i checked the Head temp in my telemetry it was down at 145 where it is normally between 180 and 200 when it is tuned and running good. I have flown it probably another 12 flights and it will do this about every 2nd or 3rd flight, I have had it come back mid-flight and run good again. It usually always starts out fine then goes rich a minute or two into the flight. It seems the fuel pressure is varying somehow. Has anybody else had this issue?

Is that a 2 piece hatori? If yes then you might want to check the o-rings on the header. . We had a similar issue that the heli went super rich when inverted only and it turned out to be the orings

It's a single piece muffler, I hadn't questioned the check valve because it always has a lot of pressure after the flight but I blew through it and it wasn't opening freely and would wistle I pulled it apart and found nothing wrong when i put it back together it seems normal and air flows freely. When it opens there is a nice poppet sound.

It's best to draw through a check valve. Blowing through it isn't the most positive way to confirm that it works. Fuel tank pressure is going up in flight. That is the only way that you are going rich on that engine. Something is changing the fuel delivery pressure so that's the first area to look at. What could that be?

TM

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Fuel pressure. This has happened to me also, I was useing another brand check valve, (wrong). Make sure you use only the YS2510 check valve, your problems should be over!! you go rich mine went lean. I lost 2 engines useing another brand!!!!!

So was this issue ever resolved?

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Also check the main needle spring clip. They are easily tweaked by a finger nail inadvertently and may no longer be applying enough pressure to keep the needle set. Happened to me on a YS 90...but it went lean. Had to rebuild it. I now put a small length of fuel tubing around the needle and the clip. Won't happen again. Mid range needle is ackbasswards with the spring clip and out of harms way.